API deficiency is a relatively common genetic disorder that predisposes affected individuals to liver disease and/or pulmonary emphysema. The most common type of API deficiency termed protease inhibitor type Z (PiZ), is transmitted as an autosomal recessive trait and affects approximately 1 in 1700 live births in most Northern European and North American populations. The PiZ mutation is a single nucleotide substitution that results in a single amino acid substitution (glutamate 342 to lysine).
Human API has 394 amino acids, one cysteine residue, and three carbohydrate side chains, giving an overall molecular weight of 52 kDa. A single reactive center loop (RCL) is located at a Met-Ser sequence at position 358-359. The tertiary structure of API contains 8 well-defined α-helices (A-H) and 3 large β-sheets (A-C). It is believed that serpins function via irreversible “suicide-substrate” mechanism. Upon cleavage by a protease, the serpin undergoes a conformational change, whereby the RCL is cleaved and inserted into the center of the A β-sheet, and the protease is translocated to the distal end of the molecule. This structural transition results in stabilization of the serpin and extensive distortion of the protease's tertiary structures, which inactivates its catalytic machinery. It is believed that the biological activity of API can be effected by, e.g., chemical modifications, including inter- or intramolecular polymerization, oxidation, complex formation, and/or cleavage by non-specific proteinases.
It is believed that the major physiological function of API is the inhibition of neutrophil elastase, cathepsin G, and proteinase 3. The API produced in individuals with PiZ API deficiency is functionally active, although there may be a decrease in its specific elastase inhibitory capacity. The predominant site of API synthesis is the liver, however, it is also synthesized in extrahepatic cell types including macrophages, intestinal epithelial cells and intestinal Paneth cells.
The pathogenesis of lung injury in API deficiency is attributable to the marked reduction in available API activity. API has been found to constitute greater than 90% of the neutrophil elastase inhibitor activity in pulmonary alveolar lavage fluid. Thus, it appears that the destructive lung disease seen in many individuals with API deficiency is due to a perturbation in the net balance between elastase and API within the lungs. The uninhibited activity of neutrophil elastase, cathepsin G, and proteinase 3, in turn, results in slow destruction of the connective tissue integrity of the lungs. This destruction of connective tissue leads to over distension and a reduction in the retractive force of the lungs which results in decreased expiratory airflow. Smoking exacerbates the problem by causing oxidative inactivation of what API is present.
At present, treatment options for individuals with pathologies associated with API deficiency are limited. Liver disease associated with API deficiency has been treated by orthotopic liver transplantation. Somatic gene therapy to replace the defective API gene has been discussed, but has yet to be successfully used.
There remains a need, therefore, for API compositions that are stable and that provide desirable API plasma bioavailability levels following administration to a subject.